The present invention relates generally to a semiconductor laser and more particularly to a semiconductor laser incorporating multi-quantum barriers for improved carrier confinement properties.
In many modern applications it is desirable to utilize efficient semiconductor laser devices. The efficiency of semiconductor laser devices may be increased by improving the optical and the carrier confinement of the semiconductor laser device. Such efficient semiconductor laser devices may be utilized in a variety of applications including semiconductor diode laser pumping of Nd:YAG lasers and optical communication, detection and illumination applications.
For efficient semiconductor laser operation it is desirable to have both optical and carrier confinement. Carrier confinement, the confinement of the holes and the electrons within the active region of the semiconductor laser device, is desirable as poor carrier confinement may cause the threshold current of the semiconductor laser device to increase and the characteristic temperature T.sub.0 to decrease. Carrier confinement in a semiconductor laser device having an active region, typically a quantum well region, is generally acheived by surrounding the active region of the semiconductor laser with materials having a large bandgap such that the carriers will be much more likely to populate the active region than they would the adjacent materials having the larger bandgap. While this method of utilizing materials having a large bandgap adjacent to the active region is successful in confining carriers, some material systems do not have readily available materials with sufficiently large bandgaps that can be grown adjacent to the active region to provide carrier confinement.
For example, an (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P semiconductor laser may be grown such that the quantum well Ga.sub.0.5 In.sub.0.5 P is positioned between a pair of graded index layers of (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P in which the aluminum mole fraction is linearly increased in a direction away from the quantum well. The bandgap is relatively small in the graded index region as the (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P semiconductor laser has a conduction band with at least two valleys. For aluminum mole fractions, x, less than 0.65, the potential barrier achievable is determined by the gamma valley. Within this transition region wherein the aluminum mole fraction is less than 0.65, the bandgap energy of the GRINSCH structure gradually increases as the aluminum mole fraction increases as is desired for carrier confinement improvement. At aluminum mole fractions greater than 0.65, the potential barrier achievable, a satellite valley, the X valley. In the X valley, however, the bandgap energy changes very little with increasing aluminum mole fractions such that the bandgap energy in this portion of the GRINSCH region does not increase substantially and thus does not significantly improve and assist the carrier confinement process. A similar situation in AlGaAs lasers occurs for aluminum mole fraction greater than 0.37.
Iga, et al in an article entitled Electron Reflectance of Multiquantum Barrier (MQB) published in Electronics Letters on Sep. 11, 1986, Volume 22, No. 19, pages 1008-1010 as well as Takagi, et al in an article entitled Potential Barrier Height Analysis of AlGaInP Multi-Quantum Barrier (MQB) published in Japanese Journal of Applied Physics, Volume 29, No. 11, in November 1990, pages L1977-L1980 and Iga, et al in U.S. Pat. No. 5,091,756 reported that alternating, thin layers of high and low bandgap materials can form an effective potential barrier that is larger than the potential of either of the barrier materials alone. The increased effective barrier height of the multi-quantum barrier is due to the interference of the electron waves which are reflected by the various barrier layers in a manner analogous to the interference of optical waves being reflected from a multi-layer dielectric coating. Additionally, Kishino, et al in an article entitled, Enhanced Carrier Confinement Effect by the Multiquantum Barrier in 660 nm GaInP/AlInP Visible Lasers, published in Applied Physics Letters, Volume 58, No. 17 on Apr. 29, 1991 on pages 1822-1824 utilized the multi-quantum barrier effect of improving carrier confinement to increase the characteristic temperature of the visible laser they were experimenting with so as to decrease the threshold current.
It should be noted, however, that each of these implementations of a multi-quantum barrier was accomplished in a semiconductor laser diode which did not include a graded index region adjacent to the active region but instead utilized confinement layers having a consistent composition therethrough. Additionally, in each of the prior multi-quantum barrier implementations the layers were lattice matched and were not strained layers.
It would be desirable to develop a semiconductor laser device in which the carrier confinement is improved such that the threshold current is decreased and the characteristic temperature is increased. Additionally, it is desirable to improve carrier confinement in graded index separate confinement heterostructure semiconductor laser devices as well as in semiconductor laser devices having strained layers within its GRINSCH laser structure.